Cloudy, With a Chance of Meteors

Earlier this year, on the chilly morning of February 15th, a thirteen-thousand-ton meteor screamed above the Ural Mountains before exploding in an airburst seventy-six-thousand feet above Chelyabinsk, Russia. While the destruction caused by the shock wave was immediately clear—over a thousand people injured, thousands of buildings damaged—the scientific fallout is still manifesting. Combined with other evidence spanning the past twenty years, new reports indicate that we should rethink our notions of how frequent, and how destructive, events like this are.

The Chelyabinsk meteor’s dramatic entrance is just one piece of a story that begins deep in our past. We live among the flotsam and jetsam of a great storm that peaked four and a half billion years ago. In that storm—a slow-motion tempest of swirling gas and dust—Earth emerged as a rocky, metal-rich glob of agglomerated matter, coated with a thin veneer of atmosphere and water. Throughout its history, our planet has suffered sporadic bombardment by the remains of that tumultuous period, from the pitter-patter of microscopic rocks that burn up in the atmosphere to gargantuan asteroids that reset the global environment.

By human standards, the largest collisions are few and far between. The last time a six-mile-wide rock hit the planet was sixty-five million years ago, when it thundered into what is now the Yucatan Peninsula, creating a hundred-mile-wide crater. The climatological fallout contributed to the mass extinction of not just the dinosaurs but also some seventy-five per cent of all animal and plant life.

Impact events, as they are known, are much more frequent with smaller objects. We can expect mile-wide asteroids as often as every few hundred thousand years or so; when it comes to rocks about ten yards across, we get hit at least once a year. For our relatively recently developed human civilization, the problem lies between these sizes: every few thousand years, a boulder a hundred yards across can hit with the explosive force of more than fifteen hundred megatons of T.N.T.—enough to wipe a small country off the map. If the smaller Chelyabinsk meteor had come in above a city like New York, it would have injured many more than the twelve hundred it did in Russia.

Despite astronomers’ herculean efforts to detect and map all the potential threats—a catalogue of so-called Near Earth Objects now stands at just over ten thousand—we’re still unsure about the number of Chelyabinsk-sized bodies and how often they hit us. So while it wasn’t very nice for the Ural region, this explosive event has provided a remarkable wealth of new data.

Three new papers, published in the journals Nature and Science, provide fresh details about the Chelyabinsk meteor. Because no one was expecting the meteor, the majority of the data collected is thanks to our species’ narcissistic tendencies. In the studies published by Nature and Science, the researchers made ingenious use of public video footage from smartphones and security cameras to reconstruct the meteor’s trajectory as it tore a hundred mile path through the dawn sky at a speed of forty thousand miles per hour. Video footage also indicated the intensity of the meteor’s light, while audio tracks revealed the timing of sonic booms and the airburst shock-wave arrival. This feat of clever detective work confirmed that the thirteen-thousand ton, twenty-yard-long object exploded with a destructive energy thirty times that of the Hiroshima atomic bomb, while recovered fragments showed that it was already highly fractured before hitting our atmosphere—a fact that allowed three quarters of it simply to evaporate at high temperature, preventing significantly greater destruction than had it remained intact all the way to the ground.

Researchers now believe that this rocky mass is likely a broken-off sibling of a mile-and-a-half-wide asteroid known as 1999 NC43, which periodically but usually benignly crosses Earth’s orbital path. This is unfortunate, because it means that other pieces of 1999 NC43 could be trailing in similar orbits, toward Earth. If these siblings are of similar size, we’ll be hard-pressed to spot them until they’re exploding in our skies. And, while previously we’d thought that objects the size of the Chelyabinsk meteor hit us once every hundred and fifty years or so, the researchers report that the methods commonly used for estimating the bulk sizes of objects hitting us may have been skewed.

Infrasound—ultra-low-frequency sound—detectors across the globe are used as part of the Comprehensive Nuclear Test-Ban Treaty’s monitoring technology, to catch illicit bomb tests. The same detectors can also pick up the pulse of atmospheric meteor events, and have been used to estimate the power and rate at which they happen. The Chelyabinsk hit was the most powerful infrasound event ever recorded by this system. With this data in hand, the scientists have dug back into the monitoring records, and have reevaluated the methodology used to estimate the power of meteor explosions. They found what appears to be a discrepancy in our previous estimates of how often objects this size will hit the planet: we’ve slightly overestimated the blast damage of individual impacts, but we’ve underestimated the risk of impact by a factor of ten, meaning that meteors like Chelyabinsk could be coming every fifteen to twenty years.

This doesn’t imply that the next dinosaur-killing giant asteroid is likely to show up soon, but it does suggest that right now, and for the past few decades, we may be and have been experiencing an anomalously high incidence of twenty-yard-long rocks hitting the Earth. This is an unexpected twist, because we’ve largely assumed that the rate of impacts by asteroids is generally constant—an equilibrium with a level of randomness is at least well-understood. Instead, this data indicates that asteroids could strike us in waves, the result of some unwitnessed breakup out in space. In the case of the Chelyabinsk object, analysis of some of the seven-hundred-plus recovered fragments suggests that this breakup could have happened a million years ago, during a period when the parent object, 1999 NC43, previously strayed too close to us. The tidal force of Earth’s gravity would have flexed this body and disrupted it, producing a family of smaller asteroids.

Does this mean that it’s time to take out asteroid insurance? Not quite. More than seventy per cent of the Earth’s surface is ocean, and our densely populated areas still represent a relatively small target area. But it should prompt us to get more serious about investing in ongoing efforts to spot these extraterrestrial objects before they hit us, and to think rationally about how to prevent that.